A standard of a wireless local area network (WLAN) technology is being developed as a standard of Institute of Electrical and Electronics Engineers (IEEE) 802.11. IEEE 802.11a and IEEE 802.11b use an unlicensed band at 2.4 GHz or 5 GHz and IEEE 802.11b provides a transfer rate of 11 Mbps and IEEE 802.11a provides a transfer rate of 54 Mbps. IEEE 802.11g applies Orthogonal Frequency Division Multiplexing (OFDM) at 2.4 GHz to provide a transfer rate of 54 Mbps. IEEE 802.11n applies multiple input multiple output-OFDM (MIMO-OFDM) to provide a transfer rate of 300 Mbps for 4 spatial streams. IEEE 802.11n supports a channel bandwidth of up to 40 MHz. In this case, IEEE 802.11n provides a transfer rate of 600 Mbps.
An IEEE 802.11af standard for defining operation of an unlicensed device in a TV whitespace (TVWS) band is currently being developed.
The TVWS band, which is a frequency band allocated to a broadcast TV, includes an Ultra High Frequency (UHF) band and a Very High Frequency (VHF) band. Specifically, the TVWS band is a frequency band which is allowed to be used by an unlicensed device under a condition that this does not hinder communication of a licensed device that operates in the frequency band. The licensed device may include a TV or a wireless microphone. The licensed device may be referred to as an incumbent user or a primary user. To overcome the problem of coexistence of unlicensed devices, a signaling protocol such as a common beacon frame, a frequency mechanism, or the like may be required for the unlicensed devices.
Operations of all unlicensed devices are permitted in a band of 512-608 MHz and a band of 614-698 MHz. However, only communication between fixed devices is permitted in bands of 54-60 MHz, 76-88 MHz, 174-216 MHz, and 470-512 MHz. The term “fixed device” refers to a device that performs signal transmission only at a fixed location. An IEEE 802.11 TVWS terminal is an unlicensed device that operates, in the TVWS spectrum, using an IEEE 802.11 media access control (MAC) layer and a physical (PHY) layer.
An unlicensed device that desires to use the TVWS band should provide a function to protect licensed devices. Accordingly, the unlicensed device should check whether or not a licensed device occupies the band before the unlicensed device starts signal transmission.
To accomplish this, the unlicensed device may check whether or not the band is being used by a licensed device by performing spectrum sensing. Examples of the spectrum sensing mechanism include an energy detection scheme and a feature detection scheme. The unlicensed device may determine that a licensed device is using a specific band when the intensity of a received signal is greater than a specific level or when a DTV preamble is detected. Upon determining that a licensed device is operating in a channel that is immediately adjacent to a channel currently used by the unlicensed device, the unlicensed device should reduce transmission power.
In the spectrum sensing mechanism, power consumption of a station (STA) is relatively high since the sensing duration is a relatively long period greater than 10 ms. Particularly, the sensing duration is proportional to the possibility of detection of a licensed device signal and therefore the possibility of detection of a licensed device signal increases as the sensing duration increases. Accordingly, there is a need to provide an efficient scanning mechanism that can reduce power consumption of the STA.